A prior art search was conducted and uncovered the following U.S. patent references: U.S. Pat. Nos. 3,273,064, 3,636,452, 3,865,990, 4,109,202, 4,159,454, 4,161,694, and 4,206,464. All of the circulators shown in the above patents are unidirectional. On the other hand, the present invention uses field reversible circulators 51 to channelize signals from two antennas 1, 2.
FIG. 2 shows one approach proposed by prior artisans for solving the space diversity problem. Antennas 1 and 2 are pointing in different directions. Receivers 3 and 4 receive the electromagnetic energy emanating from antennas 1 and 2, respectively, and pass said energy through multiplexers 5 and 6, respectively. For each channel the output of multiplexer 5 or 6 is then selected by means of a switch 31 so the signals in the N channels from the N selected ground stations (of a possible 2N) can be processed by N high power amplifiers 32.
Multiplexer 5 comprises N unidirectional circulators 11. N filters 13 are coupled to the N circulators 11, respectively, and channelize the energy therefrom into N channels 12, which are isolated by means of N isolators 14.
Similarly, multiplexer 6 comprises N unidirectional circulators 21. N filters 23 are coupled to the N circulaors 21, respectively, and channelize the energy therefrom into N channels 22, which are isolated by means of N isolators 24.
The problem with this prior art approach is that it uses a lot of excess hardware, i.e., switches 31 and an entire second set of multiplexer 6 components: circulators 21, filters 23, and isolators 24.
A second approach proposed by prior artisans is illustrated in FIG. 3. In this approach, energy is received by the two antennas 1, 2, processed by the two receivers 3, 4, and combined by means of a 3 dB coupler 9. A single output multiplexer 8 is then used to channelize the energy into N channels 42. This is done by means of N unidirectional circulators 41 and N filters 43 coupled thereto, respectively.
The problem with this prior art approach is that the signal-to-noise ratio is degraded, by 3 dB, because noise from each receiver 3, 4 is processed along with the signal. A second problem with this approach is that, for each channel 42, the station communicating to one of the antennas 1, 2 must be relied upon to be silent when the other antenna 2, 1 is receiving information for that channel 42. This is not always possible or feasible.
The instant invention remedies the above problems by means of channelizing signals from two antennas 1, 2 using just a single multiplexer 7, without significantly degrading the signal-to-noise ratio.